Surface treatment of magnesium and its alloys



Patented July 7, 1942 UNITED STATES PATENT- OFFICE SURFACE TREATMENT or MAGNESIUM AND rrs armors Herbert K. De Long,'Midland, Micla, assignor to The Dow Chemical Company, Midland, Mich, a corporation of Michigan No Drawing.

6 Claims.

sistant.

Application April 13, 1940, Serial No. 329,533

(opus-s) Briefly, the process of the invention contemplates forming a surface finishon articles of magnesium and magnesium-base alloys by subjecting the article to the action of an aqueous solution essentially comprising a water-soluble cative coatings applied thereto to provide a highly corrosion-resistant film on the metal article.

In so far as I am aware, the treating bath according to the invention may contain any watersoiuble fluoride. Hydrofiuoric acid, simple fluorides such as sodium fluoride, acid fluorides such as potassium bifluoride. and complex compounds centration of 0.2 to 7 per centby weight of hydrogen chloride in the bath is desirable. In the case of nitric acid, the preferred range is between about 4 and about 20 per cent by weight. Other acids should be used in corresponding dilutions. Concentrations of the fluoride and the acid lower than those stated are less effective in the treatment according to the invention, and concentrations higher than the preferred ranges given tend to increase the attack on the magnesium surface and to produce a much less desirable surface finish.

In carrying the inventioninto practice, the article of magnesium or magnesium-base alloy is first freed of surface. contamination, grease, etc., by wire-brushing, sanding, or chemical cleaning. Itis then subjected to the action of an aqueous fluoride-acid solution of the type described by spraying or washing the solution on the metal, or by immersing' the article in the A solution. The temperature of the treating solusuch as fluosilicic acid, fluoboric acid, and their salts, are all suitable. The bath also may comprise any a'cid chemically reactive with metallic magnesium. The common strong mineral acids, e. g., hydrochloric, nitric, and sulfuric acids, are ordinarily employed, but other acids, both inorganic and organic, e. g.,.phosphoric, acetic, chloroacetlc, and citric acids, and the like, may also aqueous solutions of these materials are by far 7 the most satisfactory in the process. The fluoride is preferably used in a concentration corresponding to between about 0.2 and about 6 per cent by weight of fluorine in the solution. If hydrochloric acld'ls employed as the acid, a contion' is not critical, ordinary room temperature being convenient. Immersion times of 0.5 to 2 minutes are customary, although somewhat longer times are not detrimental. After treatment the article is rinsed in water or in dilute aqueous alkali to remove the acid solution, and is then usually allowed to dry.

The fluoride-acid bath imparts a pleasing surface appearance and a protective coating to the article, which for many purposes may be then used without further treatment. However, the surface finish resulting from the process is especially suited to serve as a base for additional protective coatings, such as chemical and anodie coatings, electrodeposlted metal films. and particularly siccative or resinous films, such as paints and enamels. In applying these latter'films, the surface treated article is given a primer coat and then several finish coats of the paint. The resulting finished surface is highly corrosionresistant, much more than that of similar painted articles which have not been previously treated according to the invention, principally because of the improved adherence of the paint film.

The following examples will illustrate the invention, but are not to be construed as limiting itsscope:

' Example 1 Rolled sheets of a magnesium-base alloy containing 4.0 per cent aluminum and 0.3 per cent manganese, the balance being substantially magnesium, were carefully cleaned to remove grease and dirt. They were then immersed at room temperature for one minute in a water solution,

containing 1.2 per cent by weight ofhydrogen water and dried.

fluoride (HF) and 0.68 per cent of hydrogen chloride (HOD, after which they were rinsed in Example 2 The process describedin Example 1 was re peated, except that the treating 'bath was a water solution containing 3.6 per cent by weight of hydrogen fluoride (HF) and 11.6 per cent by weight of nitric acid (HNOa) Example 3 Sheets prepared as-in Examples 1 and 2, and.

similar sheets which had been cleaned but had acid.

was bent double. The untreated sheets, in contrast, showed a much poorer corrosion resistance, and the paint fllms were far less adherent.

It will be appreciated that the invention pro- Vides an unusually advantageous method of finishing and protecting the surface of articles of magnesium and. its alloys- Unlike most prior processes, only a single treatment is required; the chemicals used are inexpensive and in low concentration. No electrolytic apparatus is needed.

The process is extremely simple, rapid, and effective.

cative coating.

2. In a method of treating the surface of articles of magnesium and magnesium-base alloys, thestep which comprises subjecting the article to the action of a solution consisting of water, an acid reactive with metallic magnesium and a water soluble fluoride, the latter being employed in a concentration corresponding to between about 0.2 and about 6-per cent by weight of fluorine in the solution.

3. A process according to claim 2 wherein the acid employed is nitric acid, the same being present in a concentration between about 4 and about 20 per cent by weight of the solution.

4. A process according to claim 2 ing present in a concentration of between about 0.2 and about '1 per cent by weightof hydrogen chloride in the solution.

5. An article formed of magnesium or a magnesium-base alloy the surface of which is covered with a protective coating prepared according to the process of claim 1.

6. The method of treating the surface of articles of magnesium and magnesium-base alloys which comprises: subjecting the article to the action of a solution consisting of water, a water soluble fluoride, and a strong mineral acid; rinsing the article to remove the treating solution; drying; and covering the dried article with a sic- HERBERT K. DE LONG.

I wherein the acid employed is hydrochloric acid, the same be- 

